<style>
#container {
    border: 2px dashed #444;
    height: 125px;

    text-align: justify;
    -ms-text-justify: distribute-all-lines;
    text-justify: distribute-all-lines;

    /* just for demo */
    min-width: 612px;
}

.box1, .box2, .box3, .box4 {
    width: 150px;
    height: 125px;
    vertical-align: top;
    display: inline-block;
    *display: inline;
    zoom: 1
}
.stretch {
    width: 100%;
    display: inline-block;
    font-size: 0;
    line-height: 0
}

.box1, .box3 {
    background: #ccc
}
.box2, .box4 {
    background: #0ff
}
</style>
<div id="container">
    <div class="box1"></div>
    <div class="box2"></div>
    <div class="box3"></div>
    <div class="box4"></div>
    <span class="stretch"></span>
</div>?